1. Field of the Invention
The present invention relates to a pulse signal transmitting apparatus used in a ground station apparatus of distance measuring equipment (DME) which is one of aeronautical radio navigation aids, and more particularly, to a waveform adjusting technique for removing a distortion occurring in a pulse waveform of a transmission signal.
2. Description of the Related Art
As is generally known, DME receives an interrogation pulse signal transmitted from an aircraft in a ground station, detects a temporal position thereof, and transmits a response pulse within a predetermined period, thereby causing the aircraft to recognize distance information (see, for example, Japanese Patent No. 2629612). Here, a frequency of a transmission/reception channel is assigned to the DME in advance, and a waveform of a modulation pulse to be used is approximated to the Gaussian error function so that a bandwidth of the pulse spectrum can be made narrow. Further, a procedure of making a timing reference point for detecting a position (hereinafter referred to as a pulse position) of a pulse on the time axis a half-amplitude point of the pulse rising edge is employed. Under such circumstances, in the DME, detection accuracy of a pulse position directly exerts a great influence on the system accuracy.
Here, in the ground station apparatus of the DME, the transmission pulse signal is amplified and output by a high-output power transistor at the output stage. For this reason, the output waveform of the transmission pulse signal obtained at the output stage is inevitably distorted by the characteristics of the amplifier circuit including the power transistor. In the DME, a rise time, fall time, pulse width, and the like of a waveform of a transmitted pulse are prescribed, and when a transmission pulse waveform does not satisfy the prescription, the waveform must be adjusted so as to satisfy the prescription without fail.
It is impossible to forecast how the waveform is distorted as compared with the input waveform because the distortion depends on the characteristics of the amplifier circuit including the power transistor. For this reason, in the prior art technique, adjustment has been performed by slowly changing an input waveform while observing an output waveform, and hence a great deal of labor has been required in the adjustment work.
Incidentally, in Japanese Patent No. 2629612 described above, although automatic gain control of the DME is described, automatic waveform shaping is not described.
As described above, in the DME ground station apparatus, when the transmission pulse signal is amplified, an unpredictable distortion occurs in the signal, and hence adjustment has been performed by slowly changing an input waveform while observing an output waveform, and a great deal of labor has been required in the adjustment work.